Method for synthesizing nanoscale structures in defined locations

ABSTRACT

A method is disclosed for directly synthesizing nanoscale structures, particularly in defined locations. The method overcomes problems in nanoscale manufacturing by enabling the direct fabrication of composites useful for constructing electronic devices. In one aspect of the method, nanotubes and arrays of nanotubes are synthesized directly at defined locations useful for constructing electronic devices.

CROSS REFERENCE TO RELATED APPLICATION

This is a divisional of U.S. patent application Ser. No. 10/522,850,filed Jan. 28, 2005, now U.S. Pat. No. 7,226,663, issued Jun. 5, 2007,which is the § 371 U.S. National Stage application of InternationalApplication No. PCT/US2003/024070, filed Jul. 31, 2003, which waspublished in English under PCT Article 21(2), which in turn claims thebenefit of U.S. Provisional Application No. 60/400,897, filed Aug. 1,2002.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made in part using funds provided by the NationalScience Foundation Grant Nos. DMR-0097575 and ECS-0217061. The UnitedStates government may have certain rights in this invention.

FIELD

This disclosure concerns a method for synthesizing nanoscale structuresin defined locations, and composites and devices comprising nanoscalematerials.

BACKGROUND

Since their discovery in 1991 by microscopist Sumio Iijima, carbonnanotubes have intrigued researchers with their structures and theapplications enabled by their unique physical properties. Iijima, S.Nature, 1991, 354, 56. Nanotubes exhibit high chemical resistance andmechanical strength, among other desirable physical properties. Ongoingchallenges to exploiting these desirable properties include difficultiesassociated with isolating and manipulating nanotubes for use as discretedevice elements.

I. Carbon Nanotubes

Carbon nanotubes are graphene cylinders that can be capped at either endwith a fullerene-like structure. When he discovered nanotubes, Iijimawas analyzing materials formed at the cathode during arc dischargesynthesis of fullerenes and observed a variety of related structures,including novel, closed graphitic structures, such as nanotubes andnanoparticles. Since Iijima's initial discovery, carbon nanotubes haveattracted considerable interest from the scientific community, and haveprompted much research into their potential applications. Research alsohas been directed to other nanoscale materials, including inorganictubular materials, such as silicon carbide and tungsten sulfidenanotubes. However, the difficulties in manipulating individualnanoscale structures, such as nanotubes and connecting such structuresto other materials remains a challenging obstacle to achieving practicalapplications of these intriguing materials.

II. Applications of Nanotubes

Generally, known methods for synthesizing nanotubes provide onlyunorganized, tangled nanotubes and bundles of nanotubes. These bulkmaterials are useful as additives for improving the material propertiesof polymer or metal composites. See, U.S. Pat. No. 6,280,697, issued toZhou, et al. (Zhou). Zhou describes using bulk carbon nanotubes withintercalated lithium ions to improve the performance of lithium ionbatteries. U.S. Pat. No. 6,420,293, issued to Chang, et al. (Chang),describes using nanotubes in bulk as a filler material in ceramic metaloxides to enhance the ceramic's mechanical strength. Despite promisingproperties and successes using nanotubes as a bulk material, thedifficulties in localizing and organizing nanotubes have limited thefabrication of functional devices using nanotube components.

III. Focused Ion Beam

Focused ion beam (FIB) systems have been manufactured commercially forabout fifteen years, and primarily are used for semiconductor failureanalysis and device edit. FIB systems are similar to scanning electronmicroscopy (SEM) systems except that FIB systems use a finely focusedbeam of ions, such as gallium ions, instead of electrons. FIB systemscan be used for microscopy, micromachining and deposition processes.More recently, dual-beam systems, including both an electron beam (EB)column and a FIB column, have been developed. EB has a smaller beam spotsize than FIB, which allows the imaging of more detailed features thanFIB. See High Resolution Focused Ion Beams: FIB and Its Applications; byOrloff, J., Utlaut, M., Swanson, L. Kluwer Academic/Plenum Publisher:New York, Boston, Dordrecht, London, Moscow; 2002, which is incorporatedherein by reference.

SUMMARY

A method is disclosed for fabricating nanoscale materials, such asnanotubes, in defined locations. Also disclosed are electronic devicesfabricated according to embodiments of the method. In one embodiment ofthe method, a catalytic material is directly deposited on a substrateusing FIB-induced deposition. Nanoscale materials can then besynthesized only at the sites having catalyst. In another aspect of themethod, a material, such as a conducting, semiconducting or insulatingmaterial, is deposited on the substrate at defined locations prior tocatalyst deposition. Catalytic material can then be depositedselectively on the first deposited material, and nanoscale material canbe synthesized selectively in a pattern defined by the catalystlocation. FIB milling can be used in conjunction with EB-induced and/orFIB-induced deposition to refine deposited features, or can be usedfollowing non-selective deposition to provide a desired pattern.

Particular materials that can be deposited include, without limitationW, Pt, Au, Al, Fe, Ni, Co, Ti, Ta, Cu, and combinations thereof. Aparticularly useful metal for practicing disclosed embodiments of thepresent method for fabricating electronic devices is Pt. A second metalthat is particularly useful for fabricating a nanotube field emitterdevice is W. Exemplary useful metal catalysts and catalyst precursorsinclude Fe, Ni, Co or their combinations.

The structures synthesized can be any nanoscale structures, such asnanowires, nanotubes, nanocoils or nanobelts. The nanoscale structurestypically contain a material such as zinc oxide, silicon dioxide,tungsten oxide, cadmium sulfide, carbon, silicon carbide, or acombination thereof. Nanotubes, for example, can be any type ofnanoscale tubular materials, such as carbon nanotubes, silicon carbidenanotubes, tungsten sulfide nanotubes and other inorganic nanotubes. Inone aspect the nanotubular structures are solid materials, for exampleas wires or filaments. Such solid materials, including nanowires, can bemade from zinc oxide, silicon dioxide, tungsten oxide, cadmium sulfide,carbon, silicon carbide, or a combination thereof. Additionally, thenanotubes can be single-walled or multi-walled, and can be synthesizedso that they are oriented in any direction. For example, one compositeprepared included a substantially horizontal nanotube connecting twometal pillars. This type of composite is useful as a two terminaldevice. Another composite included a substantially vertical nanotube. Anarray containing such substantially vertical nanotubes is useful forforming a field emission device.

One aspect of the method involves synthesizing a nanotube in directelectrical contact with an electronic component. For example, nanotubescan be synthesized directly on a conducting or semiconducting electrode.Thus, an electrical connection is provided to the nanotubes without needfor tedious manipulation of nanoscale components. For example, a metalpillar can function as a conductive contact and can connect the nanotubeto a device or device component on a substrate. In another embodiment, adevice having at least two terminals can be fabricated. Particularexamples of such devices include diodes, triodes, optoelectronicdevices, acoustic wave devices, electromechanical resonators, andtransistors. In an embodiment of a transistor, a source can be providedby depositing a metal pillar on a substrate. A catalytic material can bedeposited or patterned according to the present method, such that ananotube can be synthesized with a first end contacting the pillar andextending substantially horizontally so that a second end contacts asecond pillar. The first pillar provides a source connection and thesecond pillar is connected to the second end of the nanotube, therebyproviding a drain connection. The transistor can be switched using afield effect gate, which also can be fabricated according to the presentdisclosure. Alternatively, because substantially vertical nanotubes canbe prepared according to embodiments of the method, a transistor can befabricated using a substantially vertical nanotube.

In another aspect of the method nanotube composites that function asfield emitters can be prepared. Embodiments of the method are useful forpreparing defined arrays of nanotube field emitters. Arrays havingsubstantially vertically aligned nanotubes are particularly useful forpreparing field emission devices suitable for use, for example, in aflat panel display. Such field emitters can be assembled by patterningpillars on a substrate and then patterning a catalytic material, so thatcatalytic material resides on top of the pillars. Nanotubes can then besynthesized on the pillars. Each pillar bearing a nanotube, or clusterof nanotubes, can then serve as an independent field emitter and providea single pixel in a flat panel display. Alternatively, a single pixelcan comprise a cluster of pillars bearing nanotubes. The array ofindividual nanotube field emitters may be formed on a substrate of anysize; the upper limit of such an array is only limited by the size ofthe chamber used for nanotube synthesis.

Another embodiment of a nanotube field emitter device is useful as amonochromatic electron source. In this device, a nanotube serves as ahigh brightness field emission cathode. This device can be made bysynthesizing a nanotube, such as a carbon nanotube, typically amulti-walled carbon nanotube, directly on a metal tip, examples of whichinclude W, Pt, Au, Al, Fe, Ni, Co, Ti, Ta, Cu, alloys thereof, andcombinations thereof. Such metal tips can be fabricated according to thepresent method by FIB-induced deposition and/or patterning the metalsuch that the metal tip typically has a diameter of about 1 μm or less.Tungsten is a preferred metal for fabricating high-brightness, nanotubefield emitters, where the tungsten metal tip serves as a substratehosting a carbon nanotube. This type of field emitter is useful, forexample, as an electron source in a field emission microscope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a process for synthesizing arrays ofnanotubes in defined locations.

FIG. 2A is a schematic illustrating one embodiment of a defined array ofsubstantially vertical nanotubes.

FIG. 2B is a schematic illustrating one embodiment of substantiallyhorizontal nanotubes synthesized in defined locations.

FIG. 2C is a schematic of a transistor formed using a nanotube.

FIG. 3 is a schematic illustrating direct deposition of catalyticmaterial in defined locations on a substrate, followed by nanotubesynthesis.

FIG. 4 is a schematic illustrating assembly of one example of a nanotubefield emitter device.

FIG. 5A is a graph of current vs. voltage behavior for a nanotube fieldemitter fabricated according to Example 7.

FIG. 5B is a Fowler-Nordheim plot for a nanotube field emitterfabricated according to Example 7.

FIG. 6A is a digital image illustrating a 3×4 array of Pt pillars on asubstrate, each pillar having an indentation formed by FIB milling ofthe top surface of the pillars.

FIG. 6B is a digital image illustrating a single pillar of FIG. 6A.

FIG. 7 is a digital image illustrating an array of Pt pillars ofcylindrical shape formed by FIB-induced deposition.

FIG. 8 is a digital image (scale of 500 nm provided in upper left) of abundle of nanotubes synthesized on a Pt pillar.

DETAILED DESCRIPTION

Embodiments of the present disclosure concern a method for fabricatingnew, composite materials by synthesizing nanotubes in defined locations,and devices made by the method. Synthesizing nanotubes in definedlocations facilitates production of devices that exploit their uniqueproperties.

Another method for synthesizing nanotubes is disclosed by U.S. Pat. No.6,346,189, issued to Dai, et al. (Dai '189). Dai '189 discloses thesynthesis of nanotubes on “catalyst islands” on a substrate. The methoddisclosed by Dai '189 produces catalyst islands according to amulti-step procedure that uses e-beam lithography to produce a patternedresist. A catalyst is then deposited using the patterned resist todefine sites of catalyst deposition and a “lift-off” procedure isperformed to remove the resist.

Another method is disclosed by U.S. Pat. No. 6,457,350 to Mitchell(Mitchell), which teaches a technique for placing a nickel catalyst onthe tip of a pointed tungsten wire by directional deposition andanisotropic etching. Mitchell teaches that such structures are usefulfor scanning probe microscopy.

An example of a micromanipulation method is taught by Fransen et al.Appl. Surf: Sci. 1999, 146, 312-327 (Fransen). Fransen discloses amanual method using carbon glue and micromanipulators for placing acarbon nanotube on the side of a tungsten tip to make a field emissiondevice.

In contrast, the present disclosure describes an efficient method forsynthesizing nanotubes in defined locations via direct patterning and/ordeposition of catalytic materials without using lithography or resistpatterning and removal procedures. Moreover, embodiments of thedisclosed method enable the deposition of other materials, particularlymaterials that are useful in forming electronic device components. Forexample, in one embodiment, a first material, such as a conducting orsemiconducting material is deposited in defined locations, followed bythe deposition of a catalytic material on the first material. Nanotubesare synthesized at the sites having catalyst, thereby forming anelectrical connection suitable for fabricating a nanotube-basedelectronic device, such as a field emitter, diode or transistor.

In particular examples, the first deposited material discussed above isdeposited to form a pillar on a substrate. “Pillar” is defined herein asany localized, deposited material. Pillars can be formed in any size,any geometric shape, and any pattern, with the lower size limit beingdetermined by the focused beam spot size of the FIB or electron beamsystem. For example, material can be deposited with a lateral dimensionof less than about 50 nm. The upper size limit is bounded solely by thedeposition rate and commercial need. The vertical height of pillars isreadily controlled as is known to those of ordinary skill in the art bycontrolling the rate of deposition by varying beam current, dwell timeand flow of gaseous precursor with the time of deposition at a givenrate determining the vertical height.

Pillars can be deposited and/or patterned to provide a defined array ofpillars on the substrate. Alternatively, pillars may be formed accordingto an existing pattern on the substrate, such as a pattern defined byelectronic circuitry formed on the substrate. Thus, a pillar can providean electrical connection between a nanotube and an electronic device.

Catalytic materials can be deposited on a substrate having pillars byany method, and then patterned such that the catalytic material islocalized on the pillars. In another aspect of the method, catalyticmaterial is deposited directly by FIB-induced deposition. AfterFIB-induced deposition, the catalytic material optionally can bepatterned to further define catalytic sites and define sites of nanotubesynthesis. Thus, nanotubes can be synthesized in defined locations, asdetermined by the patterning process.

In general, the term “catalytic material” is used to refer to catalystsand catalyst precursors for nanotube synthesis.

FIG. 1 illustrates a first step in an embodiment for synthesizing astructure 10 having nanotubes in defined locations. Structure 10includes a substrate 12 having pillars 14 deposited on the surface.Substrate 12 can be any solid material, and typical substrates includematerials such as silicon, silicon nitride, glass, ceramics, plastics,insulating oxides, semiconductor materials, quartz, mica, metals, andcombinations thereof. Pillars 14 can include insulating, semiconductingor conducting material and can be deposited and shaped by any method.Suitable methods for depositing such materials include photolithography,EB-induced and FIB-induced deposition. Working embodiments usedFIB-induced deposition, and in particular examples FIB milling ofdeposited material was used to further refine the location, shape andsize of deposited material. Pillars can be formed by directly depositinga metal in a defined location, or pillars can be formed bymicromachining of a material on the substrate. FIB-induced deposition ofgaseous metal precursors can be used to directly deposit a metal at adefined location. Similarly, FIB can be used to micromachine a metal onthe substrate to give pillars of a desired size at defined locations.

Using FIB and/or EB techniques, gaseous precursors can be used todeposit a variety of useful metals on a substrate, such as thoseselected from the group consisting of Al, Au, Fe, Ni, Co, Pt, W, andcombinations thereof. Gaseous precursors for EB- and FIB-induceddeposition are known to those of ordinary skill in the art. Gaseousprecursors can be selected from the group of gaseous or vaporizablematerials that deposit a desired catalytic, insulative, semiconductive,or conductive material upon contact with a focused ion beam or anelectron beam. For example, volatile organometallic precursors bearingreactive organic moieties can be used as gaseous precursors for EB- andFIB-induced deposition, with particular examples of such precursorsincluding metal carbonyls, such as W(CO)₆, and metal carbonyls of cobaltand nickel. Additional examples of volatile organometallic precursorsfor deposition include ferrocene, C₇H₇F₆O₂Au, (CH₃)₃AlNH₃, (CH₃)₃Al,C₉H₁₆Pt, C₇H₁₇Pt, TMOS and TEOS. Working examples used C₉H₁₆Pt as agaseous precursor for platinum deposition and ferrocene for irondeposition. Platinum is a particularly useful material because it is anexcellent electrical conductor, and iron is useful as a catalyst forcarbon nanotube growth.

With continued reference to FIG. 1, step 16 indicates a coating processfor depositing a layer of catalyst or catalyst precursor material onstructure 10 to give structure 20. Examples of suitable coatingprocesses include sputter-coating and spin-coating. Arrows 21 indicatethe deposition of catalyst or catalyst precursor material (catalyticmaterial) to form a layer 22 of catalytic material at least on a topportion, and perhaps substantially over pillar 14 and on a top surfaceportion of substrate 12. Coating 22 can be patterned by FIB to yieldstructure 30, which has catalyst 22 localized on substantially a topportion of pillars 14. Any technique capable of producing a desiredpattern can be used in the patterning step. In working examples, pillarswere patterned by FIB such that the width of each pillar ranged fromabout 250 nm to about 5 μm. However, pillars having smaller dimensionscan be produced using an instrument having a smaller focused beam spotsize, with current instruments capable of depositing pillars having alateral dimension of about 50 nm.

Alternatively, structure 30 can be produced directly from structure 10via step 18 by FIB-induced deposition of catalytic material 22. Afterdeposition, materials optionally can be patterned further using FIB oranother technique to further refine their features.

Steps 28 and 32 both indicate nanotube synthesis. Step 28 usesconditions that yield substantially vertical nanotubes 34 synthesized inlocations defined by sites having catalyst 22 and pillars 14 yieldingstructure 40. Step 32 represents conditions for the synthesis ofsubstantially horizontal nanotubes 36, which yield structure 50.Nanotubes 36 can be used to connect different sites on a substrate, forexample, one or more pillars 14.

Synthesis conditions can be selected such that nanotubes 34 and 36 are asingle-walled nanotube, multi-walled nanotube, or a bundle of one orboth types of nanotubes. Nanotubes can be synthesized using any suitablecatalyst and conditions that allow localization of catalytic material indefined locations.

Numerous catalysts and protocols for nanotube synthesis are known in theart, and the present method can be used with any known or futuredeveloped catalyst that can be localized by selective deposition and/ormilling. Typically, catalysts include a metal, such as Fe, Co, Ni, Ti,Cu, Mg, Y, Zn, alloys thereof, and combinations thereof. Generally, bothelemental metals and their oxides can be used to synthesize nanotubes,for example, iron, zinc and oxides of iron and zinc are useful catalystsfor nanotube synthesis. Particular catalysts and conditions for nanotubesynthesis can be selected based on the type of nanotubes desired. Forexample, Dai '526 teaches chemical vapor deposition (CVD) conditionsthat are suitable for the synthesis of predominantly single-wallednanotubes. Other conditions, such as those disclosed by U.S. Pat. No.5,500,200, issued to Mandeville, et al. (Mandeville '200), tend to yieldpredominantly multi-walled nanotubes. Mandeville '200 is incorporatedherein by reference.

Other nanotube properties that can be varied by choice of catalyst andsynthesis conditions include nanotube dimensions, such as length anddiameter, and nanotube orientation relative to the substrate. Forexample, nanotubes may be synthesized such that they are substantiallyaligned with one another, meaning that most nanotubes point insubstantially the same direction.

In working examples, nanotube diameters were from about 1.0 to about 1.8nm for single-walled nanotubes, from about 1.5 nm to about 3.6 nm fordouble-walled nanotubes and from about 10 nm to about 200 nm formulti-walled nanotubes. However, smaller-diameter nanotubes can beprepared by varying reaction conditions. For example, a highconcentration of hydrogen in the synthesis yields smaller diameternanotubes. See, Dong et al., Effects of Hydrogen on the Growth of CarbonNanotubes by Chemical Vapor Deposition. J. of Nanosci. and Nanotech.2002, 2, 155-160, (Dong), which is incorporated herein by reference.

The catalyst can be localized by milling, selective deposition or both.For example, catalytic material can be deposited by a spatiallyselective technique, such as FIB-induced deposition or a spatiallynon-selective technique, such as sputter-coating, spin-coating, physicalvapor deposition and/or electrodeposition. When a non-selective processis used, the present method confers spatial selectivity by using FIB tomill the catalytic material coating from the substrate, leavingcatalytic material only in defined locations. In selected workingexamples catalytic material was deposited by spin-coating orsputter-coating and used for nanotube synthesis following FIB milling toproduce the desired pattern.

One example of a liquid phase catalyst precursor (AlCl₃.6H₂O, SiCl₄,FeCl₃.6H₂O, MoO₂Cl₂) suitable for spin-coating the substrate is taughtby Cassell, et al., J. Am. Chem. Soc. 1999, 121, 7975, (Cassell) and byU.S. Pat. No. 6,401,526, issued to Dai et al., (Dai '526), both of whichare incorporated herein by reference. In a working example, such liquidphase catalyst precursors were spin-coated on the surface of asubstrate, and the FIB was used to pattern the coating, thereby forminga pattern of catalyst-coated area. Thus, the catalyst coating can bepatterned or milled such that the catalyst is in a defined location,such as on the top of metal pillars. Nanotubes can then be synthesizedin defined locations. In one embodiment, individual catalyst particleson the substrate surface can be imaged using a high resolution FIBmicroscope or a dual beam FIB system having an electron microscope.Because individual catalyst particles can be visualized, individualparticles can be removed. This approach enables precise catalyst millingso that a high-resolution catalyst array is produced.

FIG. 2A illustrates a field emission array, which uses nanotube fieldemitters as electron emission sources. Such nanotube field emitterarrays can be fabricated according to the method disclosed herein. Insuch an array, substrate 72 includes cathodes 74, which can optionallybe deposited and/or patterned according to embodiments of the method,connected to nanotubes 78, which are synthesized using catalyticmaterial 76. Nanotubes 78 are substantially vertically aligned anddirected at the anode (not shown), which is coated with phosphor.Nanotubes 78 preferably are synthesized under conditions that providesubstantially vertically aligned nanotubes. If a method that does notprovide selective synthesis of vertically oriented nanotubes is used,undesired nanotubes optionally can be removed via FIB-milling.

FIG. 2B depicts an array of substantially horizontal nanotubes 88synthesized using catalyst 86 and connecting pillars 84 formed onsubstrate 82.

FIG. 2C depicts a nanotube transistor device 90. Source pillar 94 anddrain pillar 97 are deposited on insulating substrate 92 to provideconnection to underlying source and drain circuitry, respectively (notshown). Gate 95 is formed by depositing a metal on the insulatingsubstrate 92 having gate circuitry (not shown). Catalytic material 96 isdeposited on source 94 and drain 97, and nanotube 98 is synthesizedtherebetween. Alternatively, catalytic material 96 is deposited on onlyone of source 94 or drain 97 prior to nanotube synthesis with thenanotube being synthesized between the source and the drain so that thesource and drain are electrically connected. Even though the site ofnanotube synthesis is defined by the catalyst locations the direction ofnanotube growth is not necessarily predetermined and thus nanotubes maybe synthesized that do not connect the two desired points. However, withrespect to FIG. 2C, for example, nanotubes that do not connect source 94and drain 97 can be removed via FIB milling so that only the nanotubeshaving the desired connectivity remain.

Device 90 is one example of a device that can be assembled according tothe method disclosed herein. Other devices will be readily apparent tothose of ordinary skill in the art in view of the present disclosure.For examples of transistors using nanotubes, see: Tans et al., Nature,1998, 393, 49; and Collins et al., Science, 2001, 292, 706-709, both ofwhich are incorporated herein by reference.

FIG. 3 illustrates second method for synthesizing nanotubes in definedlocations. With reference to structure 100, catalytic material 104 isdeposited at defined locations on substrate 102 via FIB-induceddeposition. Nanotubes can be synthesized in the locations defined by thecatalytic material 104 according to steps 106 or 108, which representprotocols for substantially vertical nanotube synthesis andsubstantially horizontal nanotube synthesis, respectively.

FIG. 4 depicts the preparation of an exemplary nanotube field emitterdevice 130. Specifically, V-shape 122 represents a tungsten filament,attached to molybdenum posts 124, inserted through ceramic base 126.Tungsten was chosen for V-shape 122 in working embodiments because it isa refractory, high melting point, relatively inert material. Moreover,tungsten can be shaped by, for example, electrochemical sharpening, togive a sharpened tip. In working embodiments molybdenum was used forposts 124, because it is a high melting point, dimensionally stable,machinable material. However, other materials having similar propertiesalso can be used for posts 124. Catalyst can be selectively deposited onthe tip of V-shaped filament 122 via FIB-mediated deposition. Nanotube132 can be synthesized on the catalyst-coated tip to yield nanotubefield emitter 130. In operation, structure 130 is a high brightness,high-aspect-ratio, field emitter, which uses nanotube 132 as an electronemission source. Using a nanotube as the field emission cathode resultsin a smaller virtual source size, and the nanotube field emitter device130 is useful, for example, as a source for a field emission microscope.Jiao et al., characterized the electron field emission properties ofsuch a field emitter. See, Mat. Res. Soc. Symp. Proc. 2002, 706,113-117, which is incorporated herein by reference.

Unless otherwise specified, nanotubes were synthesized according to thepresent procedure under CVD conditions as is known to those of ordinaryskill in the art. CVD, by itself, is a spatially non-selective process.However, features of the present method render CVD spatially selective,and in working examples, CVD is used to selectively synthesize nanotubesin defined locations. Thus, in one aspect, FIB patterning of a catalystconfers selectivity upon the synthesis of nanotubes using CVD.

Working embodiments used the following CVD procedure. First, a substratehaving a catalyst patterned thereon was inserted into the CVD reactionchamber. The reactor was evacuated by a mechanical pump, with workingembodiments evacuating to a base pressure of 3×10⁻² torr. A quartz tubeCVD chamber was used in working examples; however, any CVD reactionchamber can be used. The substrate was heated sufficiently to activatethe catalyst, such as to a temperature of 700° C. Gas purging also istypically used to facilitate catalyst activation. Gases suitable forpurging include ammonia, hydrogen, nitrogen and argon. In workingembodiments hydrogen gas was introduced into the heated reaction chamberat 325 standard cubic centimeters per minute (sccm) for 15 minutes. Thesubstrate temperature was then increased to 800° C. as measured by athermocouple. At this temperature, an admixture of acetylene andhydrogen at a volume ratio of 1:13 was introduced into the reactor.Acetylene functions as a carbon source and other carbon sources are wellknown and can be used in conjunction with features of the presentmethod. Examples of other suitable carbon sources include, withoutlimitation, methane, methanol, ethane, ethanol, ethylene and the like.Working embodiments used either methane or acetylene. Without limitationto theory, it is believed that the hydrogen gas introduced with thecarbon source acts as a diluent for the carbon source and preventscatalyst poisoning by excess carbon. When complete catalyst poisoningoccurs, typically few or no nanotubes are synthesized. When partialcatalyst poisoning occurs, other species, such as amorphous carbon isproduced. The flow rates of acetylene and hydrogen were kept at 25 sccmand 325 sccm, respectively, for 15 minutes. The total pressure in thereactor during nanotube growth was 76 torr. These specific conditionsare exemplary only and can be varied as is known to those of ordinaryskill in the art.

These conditions primarily yield multi-walled nanotubes, which arepreferred for a single nanotube field emitter device. Generally,diameters of multi-walled nanotubes ranged from about 10 nm up to about200 nm in working examples; however, multi-walled nanotubes havingdiameters of from about 8 nm up to about 1 μm can be prepared. In somecases methane was used as the carbon source for nanotube synthesis. Whenmethane was used as the carbon source, single-walled and double-wallednanotubes were prepared in the same synthesis. The diameter ofsingle-walled and double-walled nanotubes produced was in the range fromabout 1.0 to about 1.8 nm and from about 1.5 to about 3.6 nm,respectively. However, single walled nanotubes can be produced havingdiameters of from about 1 to about 10 nm and double walled nanotubes canbe produced having diameters from about 1 to about 20 nm.

EXAMPLES

The following examples are provided to illustrate certain particularembodiments of the disclosure. It should be understood that additionalembodiments not limited to those particular features described areconsistent with the following examples.

Each of the following examples was performed using a FEI FIB 611 system.

Example 1

This example describes FIB-induced deposition of Pt pillars. Images ofthe pillars prepared according to this example are shown in FIG. 7. Inthis process, the Pt pillars were deposited on a silicon substrate byinjecting a gaseous compound (C₉H₁₆Pt) via the capillary needle-sizednozzle of the gas-injection apparatus. A current of 2 picoamperes (pA)was used, the magnification of deposition was 20,000×, and the pillarswere patterned in a box radius of 0.03 micrometers (μm), using a 99%overlap within each box. The deposition time was 5:00 minutes per pillardeposited in series.

Example 2

This example describes FIB-induced deposition of Pt pillars. In thisprocess, the Pt pillars were deposited on a substrate by injecting agaseous compound (C₉H₁₆Pt) via the capillary needle-sized nozzle of thegas-injection apparatus. A current of 6 pA was used, the magnificationof deposition was 10,000×, and the pillars were patterned in a box widthof 1.0 μm with a 50% overlap within each box. To deposit the pillarsevenly, the substrate was rotated 180° relative to the source after 5:30minutes, 3:30 minutes and 1:30 minutes for each pillar deposited inseries.

Example 3

This example describes deposition and patterning of a catalyst on thepillar arrays prepared according to Example 1. The patterned substrateof Example 1 was coated with a thin layer of Co via sputter-coating for90 seconds at 32 milliamperes (mA). The Co coating was patterned via FIBsputtering (500 pA), such that Co remained only on the top pillarsurfaces. The FIB-patterned substrate was then placed in a CVD reactor,and carbon nanotubes were synthesized by the catalytic thermaldecomposition of acetylene.

Example 4

This example describes coating a substrate using a liquid catalystprecursor. The liquid catalyst precursor was prepared according to theprocedure of Cassell et al. J. Am. Chem. Soc. 1999, 121, 7975. Thecatalyst precursor contained inorganic chloride precursors (AlCl₃.6H₂O,SiCl₄, FeCl₃.6H₂O, MoO₂Cl₂), a removable triblock copolymer (P-103)serving as the structure directing agent for the chlorides, and analcohol mix (EtOH/MeOH) for dissolution of the inorganic and polymercompounds. The liquid catalyst precursor was then spin-coated on thesurface of a porous silicon substrate. A 25 microliter (μL) aliquot ofliquid catalyst precursor was deposited on the surface of the substrate.After 30 seconds the substrate was spun at 5,000 rpm for 5 seconds tospin-coat the substrate. A second 25 μL aliquot of catalyst precursorwas then delivered to the substrate while spinning for 5 more seconds at5,000 rpm. The substrate was baked at 75° C. for 15 minutes. The focusedion beam was then used to sputter the substrate surface to create apattern in which some areas remained coated with catalyst and others didnot. The substrate was placed in a CVD reactor, and carbon nanotubeswere synthesized by the catalytic thermal decomposition of acetylene.

Example 5

This example describes the preparation of a 3×4 array of Pt pillars withan indentation in the top surface of each pillar. The indentations areuseful for controlling the direction of nanotube growth.

A 3×4 array of pillars was deposited using a current of 5 pA, a dwelltime per beam step of 5.0 μs, 99% beam diameter overlap per step andusing an approximately 7 μm field of view magnification. Followingdeposition of the array, an indentation was milled in the top of eachpillar using the same conditions as the deposition, except a 0.5×0.5 μmfilled box pattern was used, and each indentation was milled for 30seconds. FIGS. 6A and 6B show the array and a representative member,respectively. Such pillars having indentations aid the synthesis ofsubstantially aligned nanotubes.

Example 6

This example describes the FIB-mediated deposition of iron for nanotubesynthesis using ferrocene as a gaseous precursor. Prior to irondeposition the silicon substrate was ultrasonically cleaned in acetonefor 15 minutes. After drying, the substrate was mounted on an aluminumsample holder using copper tape and placed into the FIB apparatus.Ferrocene powder was inserted into a gas injection crucible that isconnected to a needle placed inside the vacuum chamber of a FEI 611 FIBapparatus. The gas injection needle was aimed at the silicon substrateand the crucible was heated to 48° C. To deposit a 1 μm by 1 μm area ofiron beam current was maintained at 64 pA, the dwell time per beam stepwas 0.6 μs, the beam diameter overlap per step was 0%, and the distance(d) from needle to substrate was 100 μm. Iron pillars having variousdimensions were deposited by this procedure using different parameters.For example, 3 μm by 3 μm pillars were deposited using a beam current of500 pA, a dwell time per beam step of 0.5 μs, a beam diameter overlapper step of 0% and d was 100 μm.

Example 7

This example describes a method for assembling carbon nanotube fieldemitters by directly synthesizing carbon nanotubes on the tip of asharpened tungsten wire. Pure tungsten wire of 0.1 mm diameter waselectrochemically sharpened to a tip diameter on the order of 1 μm,using a 2.5 M solution of KOH—H₂O, with a nickel strip as a cathode. Avoltage source and a multimeter were connected to the circuit, whichpassed a current through the tungsten tip as it was suspended in thesolution. After a survey of various voltages, it was determined that themost sturdy tip geometry seems to result with 15V. At this voltage, thereaction rate between the tungsten and the KOH solution was the mostrapid. After approximately 2.5 minutes, the submerged section oftungsten fell off and the current dropped sharply, at which point thetungsten tip was removed.

The sharpened tip was spot-welded to a V-shaped tungsten wire where ithad been spot-welded to a field-emission-microscope base of ceramic orglass. This assembly would serve as the substrate for the carbonnanotube emitters.

To grow carbon nanotubes on this substrate, the sharpened tungsten tipwas carefully dipped into a liquid catalyst containing EtOH, MeOH,AlCl₃.6H₂O, SiCl₄, FeCl₃.6H₂O, MoO₂Cl₂, and P-103 (a removable triblockcopolymer). The substrate was then inserted into the chemical vapordeposition reactor. The nanotube growth was accomplished by thecatalytic decomposition of acetylene with a flow rate of 25 sccm. Thereaction chamber was kept at 76 torr and the temperature of the reactionstage was maintained at 800° C.

Field emission characteristics of field emitters made according to thisexample are provided by FIGS. 5A and 5B. The field emission experimentswere performed in a field emission microscope (FEM) system with a basepressure of ˜1×10⁻⁹ Torr. The typical pressure in the FEM chamber duringthe measurement was ˜1×10⁻⁷ to 10⁻⁸ Torr. The FEM configuration has apoint-to-plane electrode geometry. For the anode, a glass plate coveredwith indium-tin-oxide (ITO) was used, with a layer of phosphor depositedover the ITO. The distance between the nanotube emitters (cathode) andthe phosphor screen (anode) was approximately 12 cm. The tungstenfilament support (carbon nanotube emitter substrate) was attached to acurrent-regulated heating supply to clean the field emitter. The fieldemission images and the current-voltage (I-V) behaviors of the nanotubeemitters were obtained by applying a negative dc voltage up to 3,000 V.Emission current measurements were recorded using a digital dataacquisition software (Test-Point) in a personal computer, which allowsconstruction of both a time-averaged I-V response and current versustime plot at each voltage. The field emission images produced on thephosphor screen were imaged using a digital camera and recordedcontinuously on videotapes. See, Jiao et al. Mat. Res. Soc. Symp. Proc.2002, 706, 113-117.

Example 8

This example describes the synthesis of nanotubes using differentconditions and the correlation of their resulting different internalstructures with their field emission properties. Nanotubes weresynthesized on the substrates according to the CVD procedure discussedabove. The internal structures of the nanotubes were analyzed byhigh-resolution transmission electron microscopy (HRTEM), and the fieldemission characteristics of nanotubes characterized by a field emissionmicroscope.

Analysis of the internal structures of the nanotubes by HRTEM revealedthat the nanotubes synthesized using iron, cobalt and nickel with theintroduction of hydrogen during the nanotube growths had an averagediameter of about 10 nm. However, the nanotubes formed using iron andcobalt catalysts exhibited better graphitization (crystallization ofcarbon) than the nanotubes prepared using nickel. The material preparedusing iron without introduction of hydrogen gas during the synthesisincluded amorphous carbon particles as well as carbon nanotubes. Thenanotubes formed under the hydrogen-free conditions had an averagediameter of about 45 nm. Specifically, the nanotubes prepared using ironcatalysts in the presence of hydrogen comprised straight graphite layersaligned parallel to the tube axis. Moreover, little amorphous carbon wasobserved on the outer surface of the nanotubes. Field emissioncharacteristics of carbon nanotubes prepared by different catalysts arequite different. As indicated in Table 1, carbon nanotubes synthesizedwith the presence of iron catalyst exhibited a low turn-on field and alow threshold field.

TABLE 1 Type of nanotube Turn-on field Threshold field Amplification(catalyst used) (V/μm) (V/μm) factor Fe 0.35 2.8 2300 Co 0.4 3 2600 Ni 59 1500 Fe (without using 9 14 700 H₂)

Example 9

This example describes the formation of ZnO nanowires on tungstensubstrates. Such nanowires are useful as field emitter devices. Using avapor transport method, ZnO nanowires were selectively synthesized ontungsten tips and on tungsten plates. In both cases a thin film of Aucatalyst was deposited and patterned in desired locations. The angularintensity and fluctuation of the field emission current from the ZnOnanowires synthesized on tungsten tips was similar to those observed forsimilar field emitters prepared using carbon nanotubes. Aself-destruction limit of about 0.1 mA/sr for angular intensity wasobserved, and the power spectra showed a 1/ƒ^(3/2) characteristic from 1Hz to 6 kHz. See, Dong et al. Appl. Phys. Lett. 2003, 82, 1096-1098.

The present invention has been described with reference to preferredembodiments. Other embodiments of the invention will be apparent tothose of ordinary skill in the art from a consideration of thisspecification, or practice of the invention disclosed herein. It isintended that the specification and examples be considered as exemplaryonly, with the true scope and spirit of the invention being indicated bythe following claims.

1. A method for synthesizing a nanotube in a defined location,comprising: providing a substrate; depositing a catalytic material in adefined location on the substrate; and synthesizing a nanoscalestructure using the catalytic material.
 2. The method according to claim1 where providing a substrate comprises forming metal pillars in definedlocations on the substrate.
 3. The method according to claim 2 where themetal is selected from the group consisting of W, Pt, Au, Al, Fe, Ni,Ti, Ta, Cu, and combinations thereof.
 4. The method according to claim 2where the metal is Pt
 5. The method according to claim 1 where thecatalytic material comprises Ag, Au, Cu, Co, Fe, Mo, Ni, Pt, Ti, Mg, Y,Zn alloys thereof, and combinations thereof.
 6. The method according toclaim 1 where the nanoscale structure comprises a nanotube.
 7. Themethod according to claim 6 where the nanotube is a single-wallednanotube.
 8. The method according to claim 7 where the nanotube has adiameter of from about 1 to about 10 nm.
 9. The method according toclaim 6 where the nanotube is a double-walled nanotube.
 10. The methodaccording to claim 9 where the nanotube has a diameter of from about 1.5to about 20 nm.
 11. The method according to claim 6 where the nanotubeis a multi-walled nanotube.
 12. The method according to claim 11 wherethe nanotube has a diameter of from about 8 nm to about 1 μm.
 13. Themethod according to claim 1 where the catalytic material is patterned byfocused ion beam.
 14. The method according to claim 6 where synthesizingthe nanotube comprises chemical vapor deposition.
 15. The methodaccording to claim 14 where chemical vapor deposition includes using acarbon source and hydrogen in about a 1 to 13 volume ratio.
 16. Themethod according to claim 2 where the pillar has a width of from about10 nm to about 5 μm.
 17. The method according to claim 1 where thecatalytic material comprises at least one of Ag, Au and Pt and where thenanoscale structure is a zinc oxide nanowire.
 18. The method accordingto claim 1 where the catalytic material comprises at least one of Ag, Auand Pt and where the nanoscale structure is a silicon oxide nanowire.19. The method according to claim 1 where the catalytic materialcomprises at least one of Ag, Au and Pt and where the nanoscalestructure is a tungsten or tungsten oxide nanowire.
 20. A composite,comprising: a substrate; a pillar formed on the substrate at a selectedlocation comprising an insulating, semiconducting or conductingmaterial; and a nanoscale structure other than a carbon nanotube formedon the pillar.
 21. The composite of claim 20 wherein the nanoscalestructure is a nanowire, nanocoil, nanobelt, or combination thereof. 22.The composite of claim 21 wherein the nanoscale structure comprises zincoxide, silicon dioxide, tungsten oxide, cadmium sulfide, carbon, siliconcarbide, or combinations thereof.
 23. The composite of claim 1 whereinthe substrate comprises plastic, silicon nitride, quartz, or mica, orcombinations thereof, the pillar comprises Pt, Au, Fe, Ni, Ti, orcombinations thereof, and the nanoscale structure comprises zinc oxide,cadmium sulfide, silicon dioxide, or combinations thereof.
 24. Acomposite, comprising: a substrate; a pillar formed on the substrate ata selected location comprising an insulating, semiconducting orconducting material, the pillar further comprising a catalyst; and asingle nanoscale structure formed on the pillar.